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# Apache Maven Surefire — Architecture Overview


## What is Surefire?

Apache Maven Surefire is the test execution framework for Maven. It ships three plugins:

| Plugin | Purpose |
|--------|---------|
| **maven-surefire-plugin** | Runs unit tests during the `test` phase |
| **maven-failsafe-plugin** | Runs integration tests during `integration-test` / `verify` phases |
| **maven-surefire-report-plugin** | Generates HTML test reports from XML results |

Surefire supports JUnit 4 (4.12+), JUnit 5/6 (Jupiter), and TestNG (6.14.3+). <br/>
Until 3.5.x, each type was executed via a dedicated provider module. From 3.6.0 on, there is only one unified provider. <br/>
Tests execute in a **forked JVM** that communicates results back to Maven through a binary event stream protocol.

---

## Module Map

The project is a multi-module Maven reactor with 14+ modules organized in layers.

```mermaid
graph TD
    subgraph "Layer 5 — Maven Plugins"
        MSP["maven-surefire-plugin"]
        MFP["maven-failsafe-plugin"]
        MRP["maven-surefire-report-plugin"]
    end

    subgraph "Layer 4 — Shared Plugin Logic"
        MSC["maven-surefire-common<br/><i>AbstractSurefireMojo, ForkStarter, BooterSerializer</i>"]
    end

    subgraph "Layer 3 — Forked JVM Bootstrap"
        SB["surefire-booter<br/><i>ForkedBooter, PpidChecker, IsolatedClassLoader</i>"]
    end

    subgraph "Layer 2 — Extension Points"
        SEA["surefire-extensions-api<br/><i>ForkNodeFactory, reporters — plugin side</i>"]
        SES["surefire-extensions-spi<br/><i>MasterProcessChannelProcessorFactory — fork side</i>"]
    end

    subgraph "Layer 1 — Core API"
        SA["surefire-api<br/><i>SurefireProvider SPI, events, stream codec</i>"]
    end

    subgraph "Layer 0 — Foundations"
        SLA["surefire-logger-api"]
        SSU["surefire-shared-utils<br/><i>Shaded: commons-lang3, commons-io, etc.</i>"]
    end

    subgraph "Providers — Loaded in Forked JVM"
        JP["surefire-junit-platform<br/><i>unified provider for all JUnit 3+ versions</i>"]
    end

    subgraph "Shading"
        SF["surefire-shadefire<br/><i>Self-test: shades provider for Surefire's own build</i>"]
    end

    subgraph "Reporting"
        SRP["surefire-report-parser"]
    end

    MSP --> MSC
    MFP --> MSC
    MSC --> SA
    MSC --> SB
    MSC --> SEA
    SB --> SA
    SB --> SES
    SEA --> SA
    SES --> SA
    SA --> SLA
    SA --> SSU
    SB --> SSU
    MRP --> SRP
    SRP --> SLA
```

### Key relationships

- **Both plugins** (`surefire-plugin`, `failsafe-plugin`) share all logic via `maven-surefire-common`. The failsafe plugin adds pre/post-integration-test lifecycle binding and a different default report directory.
- **Providers are not compile dependencies** of the plugins. They are resolved at runtime by `SurefireDependencyResolver` and loaded in the forked JVM's classpath.
- **`surefire-shared-utils`** shades commons-lang3, commons-io, commons-compress, and maven-shared-utils into `org.apache.maven.surefire.shared.*` to prevent version conflicts with user projects.
- **`surefire-shadefire`** shades the entire `surefire-junit-platform` provider into `org.apache.maven.shadefire.*` so Surefire can use a different version of itself to run its own unit tests during its build.

---

## Forked JVM Architecture

The most important architectural concept: **tests never run in the Maven JVM**. They execute in a separate forked process.

```mermaid
sequenceDiagram
    participant User as mvn test
    participant Mojo as AbstractSurefireMojo<br/>(Maven JVM)
    participant FS as ForkStarter
    participant BS as BooterSerializer
    participant Fork as ForkedBooter.main()<br/>(Forked JVM)
    participant Prov as SurefireProvider
    participant FC as ForkClient

    User->>Mojo: execute()
    Mojo->>Mojo: Auto-detect provider<br/>(ProviderDetector)
    Mojo->>Mojo: Resolve provider classpath<br/>(SurefireDependencyResolver)
    Mojo->>FS: fork()

    FS->>BS: Serialize config to .properties file
    FS->>FS: Build JVM command line<br/>(ForkConfiguration)
    FS->>Fork: Launch process<br/>(java -cp ... ForkedBooter)

    Fork->>Fork: setupBooter()<br/>- Read .properties file<br/>- Set system properties<br/>- Connect communication channel<br/>- Start PpidChecker (orphan detection)

    Fork->>Prov: Load provider via IsolatedClassLoader
    Fork->>Prov: invoke(testSet)
    Prov->>Prov: Run tests (JUnit/TestNG/etc.)

    loop For each test event
        Prov->>Fork: Report event via RunListener
        Fork-->>FC: Binary event stream<br/>(EventChannelEncoder → pipe/TCP)
        FC-->>Mojo: Decoded Event objects<br/>(EventDecoder → ForkedProcessEventNotifier)
        Mojo->>Mojo: Update console + write XML reports
    end

    Fork-->>FC: ControlByeEvent
    FC-->>FS: Run complete
    FS-->>Mojo: RunResult (pass/fail/skip counts)
```

### Fork modes

| Mode | Behavior | Flag |
|------|----------|------|
| **forkCount=1** (default) | Single forked JVM, all test classes run sequentially | `<forkCount>1</forkCount>` |
| **forkCount=N** | N parallel forked JVMs | `<forkCount>4</forkCount>` |
| **forkCount=NC** | N × CPU cores parallel forks | `<forkCount>1C</forkCount>` |
| **reuseForks=true** (default) | Same JVM reused across test classes | `<reuseForks>true</reuseForks>` |
| **reuseForks=false** | New JVM per test class | `<reuseForks>false</reuseForks>` |

### Fork configuration variants

The command line for the forked JVM is built by one of three `ForkConfiguration` implementations:

| Class | When used | Classpath strategy |
|-------|-----------|-------------------|
| `ClasspathForkConfiguration` | Default for classpath mode | `-cp <full classpath>` |
| `JarManifestForkConfiguration` | When classpath exceeds OS limits | Creates a temp JAR with `Class-Path` manifest entry |
| `ModularClasspathForkConfiguration` | Java 9+ module path | `--module-path` + `--add-modules` |

---

## Provider Model

### old Provider Model 3.5.x

```mermaid
graph LR
    subgraph "Before — 3.5.x (5 providers)"
        M1["AbstractSurefireMojo"] --> PD1["ProviderDetector<br/>(priority-based)"]
        PD1 --> JP1["surefire-junit-platform"]
        PD1 --> TNG1["surefire-testng"]
        PD1 --> J471["surefire-junit47"]
        PD1 --> J41["surefire-junit4"]
        PD1 --> J31["surefire-junit3"]
    end
```


### new Provider Model since 3.6.0

```mermaid
graph LR
    subgraph "After — 3.6.0 (1 provider)"
        M2["AbstractSurefireMojo"] --> PD2["Simplified detection"]
        PD2 --> JP2["surefire-junit-platform"]
        JP2 --> VE["Vintage Engine<br/>(JUnit 3/4)"]
        JP2 --> JE["Jupiter Engine<br/>(JUnit 5)"]
        JP2 --> TE["TestNG Engine<br/>(TestNG)"]
    end
```

The five `ProviderInfo` implementations (`JUnit3ProviderInfo`, `JUnit4ProviderInfo`, `JUnitCoreProviderInfo`, `TestNgProviderInfo`, `JUnitPlatformProviderInfo`) are collapsed into a unified detection path that always selects `surefire-junit-platform`. Framework-specific configuration (TestNG groups, JUnit 4 categories, parallel execution) is now **mapped to JUnit Platform launcher configuration** rather than being handled by framework-specific providers.


### SurefireProvider SPI

Every test framework adapter implements `SurefireProvider` (in `surefire-api`):

```java
public interface SurefireProvider {
    // Returns test classes/suites — determines fork granularity
    Iterable<Class<?>> getSuites();

    // Executes tests; forkTestSet is null for "run all" or a Class for per-test forking
    RunResult invoke(Object forkTestSet) throws TestSetFailedException, ReporterException;

    // Graceful cancellation on timeout
    void cancel();
}
```

### Provider implementations

### 3.5.x: old implementation

| Provider | Module | Test framework | Key classes |
|----------|--------|---------------|-------------|
| **JUnit 3 + POJO** | `surefire-junit3` | JUnit 3.x, plain POJOs | `JUnit3Provider`, `PojoTestSetExecutor` |
| **JUnit 4** | `surefire-junit4` | JUnit 4.0–4.6 | `JUnit4Provider` |
| **JUnit 4.7+** | `surefire-junit47` | JUnit 4.7+ with parallel/categories | `JUnitCoreProvider`, `ParallelComputerBuilder` |
| **TestNG** | `surefire-testng` | TestNG 4.7+ | `TestNGProvider`, `TestNGExecutor` |
| **JUnit Platform** | `surefire-junit-platform` | JUnit 5, any JUnit Platform engine | `JUnitPlatformProvider`, `LauncherAdapter` |


| Framework | Before (3.5.x) | After (3.6.0) |
|-----------|----------------|---------------|
| **JUnit 3** | Supported natively | Requires JUnit 4.12+ dependency (runs via Vintage Engine) |
| **JUnit 4** | 4.0+ | **4.12+** (runs via Vintage Engine) |
| **JUnit 5** | Any | Any (unchanged) |
| **TestNG** | 4.7+ | **6.14.3+** (runs via TestNG JUnit Platform Engine) |
| **POJO tests** | Supported | **Removed** |

### Breaking Changes since 3.6.0

| Change | Impact | Mitigation |
|--------|--------|------------|
| **JUnit 3 standalone** no longer supported | Projects using only JUnit 3 must add JUnit 4.12+ dependency | Add `junit:junit:4.12` — test code unchanged |
| **JUnit 4 < 4.12** no longer supported | Upgrade to JUnit 4.12+ | Mechanical version bump |
| **TestNG < 6.14.3** no longer supported | Upgrade to TestNG 6.14.3+ | Mechanical version bump |
| **POJO tests** removed | Tests without framework annotations won't be found | Add `@Test` annotations |
| **Category expression syntax** changed | Complex boolean group expressions may behave differently under JUnit Platform tag expressions | Review and test group filter configurations |
| **Provider selection** changed | Manually configured legacy providers still work (via SPI) but auto-detection always chooses JUnit Platform | Pin surefire 3.5.x or add legacy provider as dependency |

---

#### JUnit 3 tests still work

JUnit 3 test code does not need to change. You only need to ensure your project depends on JUnit 4.12+ (which includes JUnit 3 API compatibility). The Vintage Engine executes JUnit 3 and JUnit 4 tests transparently.

#### POJO tests removed

The `LegacyPojoStackTraceWriter` and POJO test detection (`PojoTestSetExecutor`) are removed. Tests must use a recognized framework annotation (`@Test` from JUnit or TestNG).

### Group / category filtering

The custom JavaCC-based category expression parser (`surefire-grouper`) is replaced by JUnit Platform's native **tag expression** syntax. For most users, `<groups>` and `<excludedGroups>` configuration works unchanged, but the underlying evaluation engine is different. Complex boolean expressions may need review.

### Backward compatibility options

If upgrading causes issues, users have two fallback paths:

1. **Pin Surefire 3.5.x** — stay on the previous version:
   ```xml
   <plugin>
       <groupId>org.apache.maven.plugins</groupId>
       <artifactId>maven-surefire-plugin</artifactId>
       <version>3.5.4</version>
   </plugin>
   ```

2. **Use a legacy provider as a plugin dependency** (transitional):
   ```xml
   <plugin>
       <groupId>org.apache.maven.plugins</groupId>
       <artifactId>maven-surefire-plugin</artifactId>
       <version>3.6.0</version>
       <dependencies>
           <dependency>
               <groupId>org.apache.maven.surefire</groupId>
               <artifactId>surefire-junit3</artifactId>
               <version>3.5.4</version>
           </dependency>
       </dependencies>
   </plugin>
   ```

## Communication Protocol

The forked JVM communicates with Maven through a **binary event stream**. Events flow one-way: fork → Maven. Commands flow the other way: Maven → fork.

### Event types (fork → Maven)

The `ForkedProcessEventType` enum defines 21 opcodes:

| Category | Events |
|----------|--------|
| **Test lifecycle** | `testset-starting`, `testset-completed`, `test-starting`, `test-succeeded`, `test-failed`, `test-skipped`, `test-error`, `test-assumption-failure` |
| **Console output** | `std-out-stream`, `std-out-stream-new-line`, `std-err-stream`, `std-err-stream-new-line` |
| **Logging** | `console-info-log`, `console-debug-log`, `console-warning-log`, `console-error-log` |
| **Control** | `bye`, `stop-on-next-test`, `next-test` |
| **System** | `sys-prop`, `jvm-exit-error` |

### Wire format

Each event is encoded as: `:<magic>:<opcode>:<data>:` using `EventChannelEncoder` (fork side) and decoded by `EventDecoder` (Maven side). The encoding uses `ByteBuffer`-backed channels for efficiency.

### Transport channels

| Channel | Configuration | How it works |
|---------|--------------|-------------|
| **Pipe** (default) | `pipe://` | stdout/stderr of forked process |
| **TCP** | `tcp://host:port` | Socket connection, configured via Extensions SPI |

---

## ClassLoader Isolation

The forked JVM uses a layered classloader strategy to prevent classpath conflicts:

```mermaid
graph TD
    Boot["Bootstrap ClassLoader<br/>(JDK classes)"]
    System["System ClassLoader<br/>(ForkedBooter + surefire-booter + surefire-api)"]
    Isolated["IsolatedClassLoader<br/>(Provider + test classes + test dependencies)<br/><b>Child-first delegation</b>"]

    Boot --> System
    System --> Isolated
```

`IsolatedClassLoader` extends `URLClassLoader` with **child-first class loading**: it calls `findClass()` before delegating to the parent, ensuring test classes and provider classes take precedence over surefire internals.

Three classpaths are maintained:

| Classpath | Contents | Purpose |
|-----------|----------|---------|
| **surefireClasspath** | surefire-booter, surefire-api, extensions | System classloader — bootstrap |
| **testClasspath** | Test classes, test dependencies, provider | IsolatedClassLoader — test execution |
| **inprocClasspath** | Classes needed in both loaders | Bridging (e.g., `SurefireProvider` interface) |

For Java 9+, `ModularClasspathConfiguration` adds `--module-path` and `--add-reads` / `--add-opens` directives.

---

## Shading Strategy

Two modules handle classpath isolation at build time:

### surefire-shared-utils

Shades third-party libraries into the `org.apache.maven.surefire.shared` package:

| Original | Shaded to |
|----------|-----------|
| `org.apache.commons.lang3` | `org.apache.maven.surefire.shared.lang3` |
| `org.apache.commons.io` | `org.apache.maven.surefire.shared.io` |
| `org.apache.commons.compress` | `org.apache.maven.surefire.shared.compress` |
| `org.apache.maven.shared.utils` | `org.apache.maven.surefire.shared.utils` |

This prevents version conflicts when user projects depend on different versions of these libraries.

### surefire-shadefire

Shades the entire `surefire-junit-platform` provider (plus `surefire-api`, `surefire-booter`) into `org.apache.maven.shadefire.*`. This exists for one reason: **Surefire must test itself**. During the Surefire build, a different (shaded) version of the provider runs Surefire's own unit tests, avoiding conflicts with the version being compiled.

---

## Process Monitoring

The forked JVM must detect when the parent Maven process dies to avoid becoming an orphan. Two complementary mechanisms exist, configured via `ProcessCheckerType`:

### NATIVE mode (`PpidChecker`)

Periodically checks if the parent PID is still alive by running platform-specific commands:

| Platform | Command | Detection method |
|----------|---------|-----------------|
| **Linux/macOS** | `ps -o etime,pid <ppid>` | If elapsed time *decreases* between checks → PID was reused → parent is dead |
| **Windows** | PowerShell `Win32_Process` query | If creation date changes → PID was reused |

Runs on a 1-second schedule. If the parent is detected as dead → `Runtime.halt(1)`.

### PING mode

The Maven process sends periodic `NOOP` commands through the communication channel. If no command is received within 30 seconds, the forked JVM considers the parent dead and halts.

### Combined mode (`ALL`)

Both NATIVE and PING run simultaneously. Either one can trigger a halt.

---

## Extensions SPI

Surefire provides two extension points that allow customizing fork communication and reporting:

### Plugin-side extensions (`surefire-extensions-api`)

| Interface | Purpose |
|-----------|---------|
| `ForkNodeFactory` | Creates `ForkChannel` instances for fork communication |
| `ForkChannel` | Bidirectional communication channel to a forked JVM |
| `EventHandler<E>` | Processes events received from forks |
| `StatelessReporter` | Generates XML test reports |
| `ConsoleOutputReporter` | Captures console output to files |
| `StatelessTestsetInfoReporter` | Writes testset summary to console/files |

### Fork-side extensions (`surefire-extensions-spi`)

A single interface: `MasterProcessChannelProcessorFactory` — loaded via `ServiceLoader` in the forked JVM. It creates the encoder/decoder pair for communicating back to Maven. Supports `pipe://` (default) and `tcp://` transports.

---

## Reporting

Test results flow through a layered reporting pipeline:

1. **Provider** generates `ReportEntry` events (test started, succeeded, failed, etc.)
2. **`TestSetRunListener`** receives events and delegates to registered reporters
3. **`StatelessXmlReporter`** writes `TEST-*.xml` files (JUnit/Ant-compatible format)
4. **`ConsoleOutputFileReporter`** writes `*-output.txt` files with captured stdout/stderr
5. **`DefaultReporterFactory`** manages reporter lifecycle and aggregates `RunResult` across forks

When multiple forks run in parallel, each fork has its own `DefaultReporterFactory`. Results are merged by `ReportsMerger` into a single aggregate `RunResult` that determines the build status.

The `maven-surefire-report-plugin` reads the XML files (via `surefire-report-parser`) and generates HTML reports — it operates post-build and is independent of the execution pipeline.

---

## Group / Category Filtering

Since 3.6.0, test filtering by groups, categories, or tags is handled via the JUnit Platform's native **tag expression** syntax.

### How it works

1. Users configure `<groups>` and `<excludedGroups>` in the plugin configuration
2. For **JUnit 4**: `@Category` annotations are mapped to JUnit Platform tags via the Vintage Engine
3. For **TestNG**: groups are mapped to JUnit Platform tags via the TestNG Engine
4. For **JUnit 5/6**: `@Tag` annotations are used natively

Tag filter expressions are passed directly to the JUnit Platform Launcher.

---

## Build and Test

```bash
# Full build with unit tests
mvn clean install

# Full build with integration tests
mvn clean install -P run-its

# Build a single module
mvn clean install -pl surefire-booter

# Run a single test
mvn test -pl surefire-booter -Dtest=ForkedBooterTest

# Run a single integration test
mvn verify -pl surefire-its -Prun-its -Dit.test=JUnit47RedirectOutputIT

# IDE setup (required before importing)
mvn install -P ide-development -f surefire-shared-utils/pom.xml
mvn compile -f surefire-grouper/pom.xml
```

**Requirements**: Maven 3.6.3+, JDK 8+ (source level 8, `animal-sniffer` enforces Java 8 API).

---

## Stack trace filtering

A new `StackTraceProvider` optimizes memory usage by truncating stack traces to 15 frames and filtering JDK packages by default. This is configurable:

```xml
<configuration>
    <stackTraceFilterPrefixes>
        <prefix>org.springframework.</prefix>
        <prefix>org.junit.</prefix>
    </stackTraceFilterPrefixes>
</configuration>
```

When not specified or empty, the default filters (`java.`, `javax.`, `sun.`, `jdk.`) apply.

---

## Stack Trace Memory Optimization

### Problem

To associate console output with test classes, Surefire captures stack traces for every output line. With full stack traces (25–30 frames typical), this consumed 600–1,800 bytes per line.

### Solution

The new `StackTraceProvider` class (in `surefire-api`) introduces:

- **Frame limit**: Maximum 15 frames per stack trace (sufficient to capture the test class after surefire framework frames)
- **Package filtering**: JDK packages (`java.`, `javax.`, `sun.`, `jdk.`) filtered by default
- **Configurable prefixes**: Users can specify custom filter prefixes that **replace** (not add to) the defaults

### Memory impact

| Metric | Before | After |
|--------|--------|-------|
| Frames per trace | 25–30 | ≤15 |
| Bytes per output line | 600–1,800 | 300–600 |
| Estimated savings | — | ~50% |

### Configuration

```xml
<!-- Custom prefixes (replaces defaults) -->
<configuration>
    <stackTraceFilterPrefixes>
        <prefix>org.springframework.</prefix>
        <prefix>org.junit.</prefix>
    </stackTraceFilterPrefixes>
</configuration>
```

```bash
# Command line
mvn test -Dsurefire.stackTraceFilterPrefixes=org.springframework.,org.junit.
```
